Over-release under objc ARC with -Os, but not with -O0

I've already filed a radar (rdar://12311693, http://openradar.appspot.com/12311693) for the following issue, but I thought I'd post here as well to see if anyone can spot a mistake in my code that could lead to the crash.

The following code sample leads to a crash due to an over-release when built with compiler optimization turned on (-Os), but does not crash when compiler optimization is turned off (-O0). The project is being built with Xcode 4.4.1 (4F1003), Apple LLVM compiler 4.0

The app crashes when num2 is over-released. Enable Zombie Objects to confirm that this is the case.

// This crashes under -Os, but not under -O0
NSNumber *num1 = @((float)arc4random() / (float)UINT32_MAX);
NSNumber *num2 = @((float)arc4random() / (float)UINT32_MAX);

NSNumber *foo1 = num1;
NSNumber *foo2 = num2;

for (NSUInteger i=0; i<2; i++) {

    NSLog(@"foo1: %p %@", foo1, foo1);
    NSLog(@"foo2: %p %@", foo2, foo2);

    // swap foo1 and foo2
    foo1 = num2;
    foo2 = num1;
}